Rotary atomizers are a type of liquid spray coating device which includes an atomizer head rotatable at high speed (typically 10,000-40,000 revolutions per minute) by an air turbine motor to apply liquid coating material, such as paint, in atomized form onto the surface of a workpiece. The atomizer head is usually in the form of a disc or cup which includes an interior wall that defines a cavity and terminates in an atomizing edge. Liquid coating material delivered to the interior of the cup flows outwardly under centrifugal force along the interior wall of the cup and is expelled radially outward from the peripheral edge of the cup to form a spray pattern of atomized droplets of coating material. To improve the transfer efficiency of the coating process, an electrostatic charge is imparted to the coating material so that the pattern of atomized coating material is attracted to an electrically grounded workpiece.
An example of an electrostatically charged rotary atomizer is disclosed in commonly assigned U.S. Pat. No. 4,887,770 ('770) to Wacker et al., which is expressly incorporated herein in its entirety by reference. In the FIG. 12 embodiment of the '770 patent, the cup (20) is made from an insulative material and includes a semi-conductive ring (546) which is charged through posts (504) by three external electrode probes (462). This system suffers from a drawback in that the front end of the housing from which the cup protrudes has a large profile that causes the air currents, generated by the high speed rotation of the cup, to create a vacuum around the front end of the housing which in turn causes the paint to wrap back onto the housing. Also, there is a need to shape the pattern of atomized coating material being sprayed from the rotary atomizer. The first problem has been addressed by directing auxiliary air from a first source of auxiliary air around the front end of the housing to break up the vacuum and thereby prevent paint wrapback. The second problem was addressed by directing auxiliary air from a second source of auxiliary air around the cup for shaping the pattern of atomized coating material being sprayed from the rotary atomizer. The need to provide two separate sources of air complicates the construction of the atomizer and can reduce the effectiveness of each air flow when the two air flows intermingle with each other. Thus, there still exists a need for an atomizer that further reduces or eliminates wrapback and does not require two separate flows of air to be directed toward the cup for breaking up the vacuum and shaping the material being sprayed.
Prior to the '770 patent, one of the hazards associated with the use of the conductive atomizing cup was the possibility of operator shock or ignition of combustible coatings because of the high voltage at which the cups were maintained. For example, as disclosed in U.S. Pat. No. 4,369,924, a charge is transferred through a turbine shaft from a power supply to the rotary atomizer cup. Since, both the cup and the entire rotary atomizing housing are metal and are charged to a high voltage, there is a significant safety hazard since the atomizer carries sufficient charge to severely shock an operator. Therefore, protective fences and interlocks have to be installed around the atomizer.
The '770 patent, listed before, discloses a low capacitance, rotary atomizer which, while electrostatically charging the coating paint at the rotary atomizer cup, does not store sufficient charge to present a shock hazard and therefore does not have to be protected by fences and safety interlocks. To charge the atomizer in the '770 patent, external electrode probes (462) direct the charge into the cup (20). However, since the cup (20) is charged through external electrode probes (462), the system suffers from the drawback that the front end of the housing has a large profile which causes the attendant wrapback problems discussed before.
Another problem associated with prior art rotary atomizers is that the rotary atomizer cups have not been easy to disassemble and clean. For example, in U.S. Pat. No. 4,838,487, a deflecting member (28) is held in place against atomizing bell (10) by spacers (36). However, in operation, dried paint can collect on the front surface (30) of the deflector member. Then, the flow of paint across the front surface with the dried paint has a tendency to form an irregular coating on the part being sprayed.
In operating rotary atomizers, an important control parameter is the speed of the air turbine. The measurement of this speed is typically accomplished with a fiber optic cable. The rear surface of the air turbine disk is colored so that one half of the surface is black and the other half silver. The difference between the two colors is sensed with a fiber optic transceiver and a signal output through a fiber optic cable to a control unit. In the control unit, the signal can be conditioned to determine the speed in revolutions per minute (RPM) of the air turbine disk. The problem with this design is that the fiber optic cable cannot withstand extended cyclical flexing (to which it is subjected during operation in a manufacturing plant) for a long enough period of time and tends to break. Also, fiber optic cable is normally encased in a sheath that cannot provide high voltage isolation required in the presence of an internally located power supply. Still another problem with the prior art designs is that the fiber optic transceiver cannot be quickly disconnected from and reconnected to the rotary atomizer without recalibration.
During the operation of the rotary atomizers, the paint can collect on the front surface of the rotary atomizer member and sometimes flow back into the atomizer device through the space formed between a stationary paint tube and the rotating turbine shaft and ultimately migrate into the atomizer device causing it to malfunction by problems such as clogged bearings.